Bushings make it possible to pass electrical conductors through an area having a different potential, and in the process to isolate these two components from one another. In particular in medium-voltage assemblies which are made up of two or more switching panels it is also desirable for bushings between the switching panels to be designed such that, in the event of an arcing fault, the effects of this fault remain limited to a specific switching panel. Bushings of this type must therefore be capable of withstanding the pressure produced in the event of an arc in order to prevent the conductors from bending, resulting in an increase in the size of the clearance, or in order to prevent the arc from migrating to other switching panels.
Conventionally, bushings comprise a fastening plate, which is made of metal or insulating material, and a cast-resin section having bushing openings. The cast-resin section serves the purpose of dissipating the electrical field, and of extending the creepage path, which extends over the surface of the cast-resin section, from the busbar to the encapsulating housing of the assembly. However, such bushings which are previously known have the disadvantage that the cast-resin section is not connected in a force-fitting manner to the fastening plate. Owing to this lack of mechanical robustness, the repulsive or attractive forces occurring in the event of a short circuit between the current-carrying conductors can no longer be absorbed. According to the prior art, cast-resin supports are therefore provided for the purpose of improving the support of the cast-resin section on the fastening plate. However, these additional components increase the manufacturing, maintenance and assembly complexity of bushings.
It has widely been known to use a bushing which has been manufactured from a plastic material and has elevations for the purpose of increasing the size of the creepage path. In order to absorb the forces occurring in the event of a short circuit, this previously known bushing is also provided with cast-resin supports which make the production, assembly and maintenance of the bushing more complicated.
DE 36 39 626 A1 discloses a bushing manufactured from rubber having a fastening flange for the purpose of attaching the bushing to a wall of a switching panel in medium-voltage switchgear assemblies. The bushing disclosed here comprises an insulating body which has an axial cavity which is open on both sides for accommodating an electrical conductor in the form of a rod. Annular outer ribs are provided for the purpose of increasing the creepage path between the conductor and the wall, which is at a different potential. Integrally formed on the side of the flange section which is remote from the annular outer ribs is an annular screen in the form of a collar, which radially delimits an end-side recess in the flange and likewise serves the purpose of increasing the creepage path. However, this described bushing has, on both sides of the fastening flange, extensions extending along the conductors to be accommodated, with the result that, owing to the bulkiness of the bushing, the components adjacent to the wall can only be mounted with a clearance.
DE 35 15 421 discloses a bushing which is composed of an inner external cone and an outer external cone. Provided within these external cones is an axial cavity for accommodating an electrical conductor. This bushing also extends on both sides of a wall, through which the electrical conductor is intended to be passed.